1. Field of the Invention
The present invention generally relates to optical disk drives, and more particularly to an optical disk drive which reproduces information from an optical disk.
2. Description of the Related Art
An optical disk drive which reproduces information an optical disk such as a CD-ROM (Compact Disk Read Only Memory) is required to rotate the optical disk at a high speed in order to increase the data reproduction speed. However, when the rotational speed of the disk is as high as a 24-times speed (the normal rotational speed is defined as 1-time speed), vibration in the optical disk due to the eccentricity and unbalance (eccentric center of gravity) of the optical disk is increased in proportion to the power of the angular speed. The above vibration affects focus servo and tracking servo, and thus data may not be read from the disk.
It is possible to improve the anti-vibration performance by increasing the tracking servo gain. However, the vibration of the optical disk drive is propagated through a casing of the optical disk drive, and is transferred to a personal computer equipped with the optical disk drive. Hence, the body of the personal computer is vibrated. Further, mechanical noise may be caused. These problems make the user uncomfortable.
A device shown in FIG. 1 is proposed taking into account the above. The disk shown in FIG. 1 includes a main board 6, on which a spindle motor 2 for rotating an optical disk, an optical pickup unit 4 and electronic components are mounted. A shock sensor 8 is attached to the main board 6, and senses a vibration caused therein. Hence, the rotational speed of the spindle motor 2 is controlled.
As shown in FIG. 2, a vibration sense signal output by the shock sensor 8 passes through an equivalent circuit 10, which outputs a signal component within a given frequency range. The signal component thus extracted is compared with a threshold voltage by a comparator 12. When the signal component exceeds the threshold voltage, the comparator 12 outputs a high-level signal, which is supplied to a microcomputer 14. In response to receipt of the high-level signal from the comparator 12, the microcomputer 14 supplies a control signal to a spindle motor driver so that the rotational speed of the spindle motor 2 is decreased. Hence, the vibration of the optical disk 1 can be reduced.
However, the comparing operation using the threshold voltage in the comparator 12 enables only the selective control as to whether the rotational speed of the optical disk should be reduced. Further, the rotational speed of the optical disk may excessively be reduced or may insufficiently be reduced. Hence, data cannot be read rapidly and/or correctly.
Further, the critical vibration which reduces the rotational speed of the optical disk depends on users' demands, and depends on how the optical disk drive is attached to the casing of the personal computer. Hence, the single threshold voltage applied to the comparator 12 does not follow changes of the critical vibration due to the users' demands and attachment.